1. Field of the Invention
Embodiments of the present invention relate to apparatus and methods for forming a wellbore, lining a wellbore, and circulating fluids in the wellbore. Particularly, the present invention relates to apparatus and methods for cementing a wellbore formed by drilling with casing. More particularly, embodiments of the present invention also relate to apparatus and methods for stage cementing a wellbore.
2. Description of the Related Art
In the drilling of oil and gas wells, drilling with casing is a method of forming a borehole with a drill bit attached to the same string of casing that will line the borehole. In other words, rather than run a drill bit on smaller diameter drill string, the bit is run at the end of larger diameter casing that will remain in the wellbore and be cemented therein. Because the same string of casing transports the bit and lines the borehole, no separate trip out of or into the wellbore is necessary between the forming of the borehole and the lining of the borehole. Drilling with casing is especially useful in certain situations where an operator wants to drill and line a borehole as quickly as possible to minimize the time the borehole remains unlined and subject to collapse or the effects of pressure anomalies. For example, when forming a sub-sea borehole, the initial length of borehole extending from the sea floor is much more subject to cave in or collapse than the subsequent sections of borehole. Sections of a borehole that intersect areas of high pressure can lead to damage of the borehole between the time the borehole is formed and when it is lined. An area of exceptionally low pressure will drain expensive drilling fluid from the wellbore between the time it is intersected and when the borehole is lined. In each of these instances, the problems can be eliminated or their effects reduced by drilling with casing.
After drilling to a predetermined depth, a cementing operation is performed. The cementing operation fills the annular space between the outer diameter of a casing and the earth with cement. The cement will set the casing in the wellbore and facilitate the isolation of production zones and fluids at different depths within the wellbore. Currently, cement flows into the annulus from the bottom of the casing (e.g., cementing the long way) or the top of the casing (e.g., reverse cementing). Due to weak earth formations or long strings of casing, cementing from the top or bottom of the casing may be undesirable or ineffective. When circulating cement into the annulus from the bottom of the casing, problems may be encountered as the cement on the outside of the annulus rises. For example, if a weak earth formation exists, it will not support the cement. As a result, the cement will flow into the formation rather than up the casing annulus. When cementing from the top of the casing it is often difficult to ensure the entire annulus is cemented.
There is, therefore, a need for apparatus and methods of cementing the drilling casing of a drilling with casing operation. There is also a need for apparatus and methods of cementing a casing string at intermediate points. A need also exists for cementing a casing string at intermediate points using a full bore stage tool.